Re: [PATCH] mm, THP, swap: fix allocating cluster for swapfile by mistake

[Gao Xiang]

Hi Andrew,

On Wed, Aug 19, 2020 at 01:05:06PM -0700, Andrew Morton wrote:
> On Thu, 20 Aug 2020 03:56:13 +0800 Gao Xiang <hsiangkao@xxxxxxxxxx> wrote:
> 
> > SWP_FS doesn't mean the device is file-backed swap device,
> > which just means each writeback request should go through fs
> > by DIO. Or it'll just use extents added by .swap_activate(),
> > but it also works as file-backed swap device.
> 
> This is very hard to understand :(

Thanks for your reply...

The related logic is in __swap_writepage() and setup_swap_extents(),
and also see e.g generic_swapfile_activate() or iomap_swapfile_activate()...

I will also talk with "Huang, Ying" in person if no response here.

> 
> > So in order to achieve the goal of the original patch,
> > SWP_BLKDEV should be used instead.
> > 
> > FS corruption can be observed with SSD device + XFS +
> > fragmented swapfile due to CONFIG_THP_SWAP=y.
> > 
> > Fixes: f0eea189e8e9 ("mm, THP, swap: Don't allocate huge cluster for file backed swap device")
> > Fixes: 38d8b4e6bdc8 ("mm, THP, swap: delay splitting THP during swap out")
> 
> Why do you think it has taken three years to discover this?

I'm not sure if the Redhat BZ is available for public, it can be reproduced
since rhel 8
https://bugzilla.redhat.com/show_bug.cgi?id=1855474

It seems hard to believe, but I think just because rare user uses the SSD device +
THP + file-backed swap device combination... maybe I'm wrong here, but my test
shows as it is.

Thanks,
Gao Xiang

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